Non-destructive inspection method

ABSTRACT

A non-destructive inspection includes: magnetizing a target by a first magnetostatic field (S 2 ); shutting off the magnetostatic field (S 3 ); measuring, at measurement points, the transient change of a differential magnetic flux density of a first residual magnetic field of the target (S 4 ); obtaining a first time constant by the main time constant of the transient change at each measurement point (S 5 ); magnetizing the target by a second magnetostatic field (S 2 ); shutting off the second field (S 3 ); measuring, at the measurement points, the transient change of a differential magnetic flux density of a second residual magnetic field of the target (S 4 ); obtaining a second time constant by the main time constant of the transient change for each measurement point (S 5 ); and obtaining information about the internal structure of the target by the distribution differences between the first and the second time constants at the measurement points (S 7 ).

TECHNICAL FIELD

The present invention relates to a non-destructive inspection method fordetermining e.g. the welding quality of a spot weld section.

BACKGROUND ART

Spot welding is a known welding technology for welding together metalplates, which can be used in the manufacture of automobiles, domesticelectrical products, and the like. In spot welding, firstly, asillustrated in FIG. 18, two superimposed metal plates 100 a, 100 b aresandwiched between a pair of electrodes 150 a, 150 b. In this state,pressure is applied locally to the metal plates 100 a, 100 b by means ofthe pair of electrodes 150 a, 150 b, and current is passed between theelectrodes 150 a, 150 b. The current flows in a concentrated mannerthrough the portion of the metal plates 100 a, 100 b sandwiched betweenthe electrodes 150 a, 150 b, and therefore generates Joule heat. Aportion of the metal plates 100 a, 100 b is melted by this Joule heat,whereupon, the passage of current is halted. When the molten portion ofthe metal plates 100 a, 100 b cools and solidifies, the metal plates 100a, 100 b will be welded together.

FIG. 19 is a cross-sectional view of a spot welded section of two metalplates 100 a, 100 b which have been spot welded as described above. Inthe spot welded section, the outer surfaces of the metal plates 100 a,100 b are dented due to the pressure applied by the electrodes 150 a,150 b. This denting is called an “indentation” 101, and the length L1thereof is called the “indentation diameter”. A nugget section 102 and apressure bonded section 103 are formed in the spot weld section. Thenugget section 102 in a region where the metal plates 100 a, 100 b havebecome unified as a result of being melted due to the application ofheat and pressure, and then solidifying. The length L2 of the nuggetsection 102 is called the “nugget diameter”. This nugget diameter L2greatly influences the welding strength achieved in the spot weldsection. The greater the nugget diameter L2, the greater the weldstrength of the spot weld section. The pressure bonded section 103 is aregion which has received the effects of the applied heat and appliedpressure and where the metal plates 100 a, 100 b have bonded togetherunder pressure. The total length L3 of the nugget section 102 and thepressure bonded section 103 is called the joint diameter. The pressurebonded section 103 is surrounded by a thermally annealed heat affectedzone (HAZ) 104. The HAZ 104 has a length L4 called the HAZ diameter. TheHAZ 104 is surrounded by an original metal 105 whose metallographicstructure has not been affected by the spot welding.

Generally, the nugget diameter L2 or the joint diameter L3 in the spotweld section achieved by welding is appropriately 10 millimeters orless, which is relatively small. Therefore, in many cases, it isnecessary to inspect the spot weld section in order to check that it hassufficient weld strength. Since the weld strength of the spot weldsection is greatly influenced by the nugget diameter L2, then the nuggetdiameter L2 can be used effectively as a basis for judging whether ornot the spot weld section has a suitable welded state.

Japanese Patent Application Laid-open No. Hei10-26609 disclosesinspection technology, one object of which is to measure the nuggetdiameter L2 in a non-destructive manner, and to judge the suitability orunsuitability of the welded state of a spot weld section on the basis ofthese measurement results. According to this patent publication, anexcitation coil is disposed in the vicinity of an inspection target, anda loop coil forming a sensor is disposed between the inspection targetand the excitation coil. In this state, a static magnetic field isgenerated which passes through the inspection target and the sensor, bypassing a DC current through the excitation coil. Thereupon, when thestatic magnetic field is shut off, the inductance of the loop coil (or aphysical quantity that is directly proportional to the inductancethereof) is determined by tracing the course of the loss of theelectrical field remaining in the inspection target. This inductanceindicates the magnetic permeability of the nugget section 102 andpressure bonded section 103, or the like, constituting the spot weldingsection through which the residual magnetic field passes. Whenmeasurement of this kind is carried out in a plurality of positions withrespect to the inspection target, then variation will occur in theplurality of inductances obtained. This variation in inductance reflectsvariations in the internal structure of the spot weld section.Therefore, the nugget diameter L2 can be estimated by detecting thevariations in magnetic permeability, and hence the variations ininductance, caused by changes in the internal structure of the spot weldsection, by means of non-destructive inspection technology.

According to the Japanese Patent Application Laid-Open No. 10-26209, thesensor loop coils are arranged to face the indentation 101 at apredetermined distance from the metal sheet 100 a or the metal sheet 100b in the non-destructive inspection. Under normal circumstances, air ispresent at the indentations 101. The magnetic permeability of the air ismuch smaller than the magnetic permeability of the nugget section 102 orthe pressure bonded section 103. Accordingly, the measured loop coilinductance reflects not only the magnetic permeability of the internalstructure of the spot weld section but also the magnetic permeability ofthe air at the indentations 101. If the inductance (or a physicalquantity proportional to the inductance) reflects the magneticpermeability of elements other than the internal structure of the spotweld section, the non-destructive inspection of the spot weld sectionmay not give an accurate estimation on the nugget diameter L2.

DISCLOSURE OF THE INVENTION

It is an object of the present invention to provide a non-destructiveinspection method capable of giving reliable inspection results in aninspection for e.g. the welding quality of a spot weld section.

According to a first aspect of the present invention, there is provideda non-destructive inspection method including: a step of magnetizing aninspection target by applying a first magnetostatic field to the target;a step of shutting off the first magnetostatic field and measuringtransient change in a differential magnetic flux density of a firstresidual magnetic field passing through the magnetized target, themeasuring being performed at a plurality of measurement positions; astep of obtaining a first time constant provided by a main time constantof the transient change for each of the measurement positions; a step ofmagnetizing the target by applying a second magnetostatic field to thetarget; a step of shutting off the second magnetostatic field andmeasuring transient change in a differential magnetic flux density of asecond residual magnetic field passing through the magnetized target,the measuring being performed at each of the measurement positions; astep of obtaining a second time constant provided by a main timeconstant of the transient change for each of the measurement positions;and an information obtaining step of obtaining information about aninternal structure of the target based on a difference betweendistribution of the first time constant and distribution of the secondtime constant at the measurement positions.

Preferably, the measurement positions may be in a row facing the target.

Preferably, the information about the internal structure in theinformation obtaining step may be obtained based on a ratio functionwhich is derived from a distribution function of the first time constantwith the measurement position as a variable and a distribution functionof the second time constant with the measurement position as a variable.Alternatively, the information about the internal structure in theinformation obtaining step may be obtained based on a differencefunction which is derived from a distribution function of the first timeconstant with the measurement position as a variable and a distributionfunction of the second time constant with the measurement position as avariable.

According to a preferred embodiment, the inspection target may be a spotweld section in a jointed plate member made by spot welding two sheetmetals. In this case, the information obtained in the informationobtaining step may comprise information about the shape of nuggetsection included in the spot weld section.

According to a second aspect of the present invention, there is providedanother non-destructive inspection method. This method comprises: ascanning step including a cycle of magnetizing a target by applying amagnetostatic field to the target, shutting off the magnetostatic fieldto measure transient change in a differential magnetic flux density of aresidual magnetic field which passes through the magnetized target at aplurality of measurement positions, and obtaining a main time constantof the transient change for each of the measurement positions, the cyclebeing repeatedly performed for each of a plurality of magnetostaticfields of different magnetic flux densities; and an analyzing step ofanalyzing a change that the main time constant undergoes at eachmeasurement position as the plurality of magnetostatic fields arechanged in the scanning step; and an information obtaining step ofobtaining information about an internal structure of the target based onan analysis result obtained by the analyzing step.

Preferably, in the analyzing step, at each of the measurement positions,a magnetic flux density of a critical magnetostatic field may bedetermined, for which field the change of the main time constant duringthe scanning step with respect to changes in the magnetostatic fieldachieves a maximum value. Further, in the information obtaining step,information about the internal structure of the target may be obtainedbased on a distribution function of the critical magnetostatic fieldwith the measurement positions as a variable.

Preferably, the measurement positions may be in a row facing the target.

Preferably, the target may be a spot weld section in a jointed platemember made by spot welding two sheet metals. In this case, theinformation about the internal structure obtained in the informationobtaining step may comprise information about the shape of a nuggetsection included in the spot weld section.

Reference is now made to FIG. 1 through FIG. 8 to describe theprinciples based on which the time constant used in the presentinvention is calculated. As specifically described below, amagnetostatic field is applied to the inspection target and then shutoff. The magnetized inspection target provides a residual magneticfield. At a predetermined position or positions, the transient change ofthe differential magnetic flux density of the residual field ismeasured, so that the time constant of the transient change iscalculated.

FIG. 1 and FIG. 2 are schematic views showing the general composition ofa device for carrying out non-destructive inspection using theapplication and shutting off of a static magnetic field, and alsoshowing the operation of this device. The non-destructive inspectiondevice comprises an excitation coil 1 wound about an iron core 2, adrive circuit 3 for driving the excitation coil 1, and a plurality ofsensor coils 4. The drive circuit 3 incorporates a DC power supply 3 a,a switch 3 b and a resistance 3 c. The sensor coils 4 are loop coils.When carrying out inspection, this device is positioned in the vicinityof the inspection target. In FIG. 1 and FIG. 2, the device is positionedin the vicinity of the spot weld section of a steel plate member 110,formed by spot welding of two steel plates. A nugget section 102 existsinside this spot weld section.

As shown in FIG. 1, when the switch 3 b is turned on, a static magneticfield F1 is applied to the spot weld section. More specifically, theswitch 3 b is turned on, a voltage output by the DC power supply 3 a isapplied to the excitation coil 1, and a DC current flows in theexcitation coil 1, whereby a static magnetic field F1 is createdsurrounding the excitation coil 1. A portion of the static magneticfield F1 is formed inside the steel plate member 110. The location inwhich the magnetic field is formed in the steel plate member 110, inother words, the location through which the magnetic flux passes, ismagnetized in accordance with the intensity of the magnetic field. Inorder to judge the suitability or unsuitability of the state of thewelding in the spot weld section, on the basis of the size of the nuggetsection 102, the non-destructive inspection device is positioned in sucha manner that the magnetic flux passes through the nugget section 102when the static magnetic field is applied.

As shown in FIG. 2, when the switch 3 b is turned off, the staticmagnetic field F1 is shut off. More specifically, when the switch 3 b isturned off, the DC current that has been flowing in the excitation coil1 until that point is shut off, and therefore the static magnetic fieldF1 is also shut off. Due to the shutting off of the static magneticfield F1, the loops of magnetic flux of the static magnetic field F1separate into a closed loop C1 of magnetic flux in the regionsurrounding the excitation coil 1, and a closed loop C2 of magnetic fluxin the region surrounding the steel plate member 110. The closed loop C1rapidly declines and disappears. By contrast, the closed loop C2 doesnot disappear immediately, but rather declines in a gradual fashion, dueto the sustaining action of the magnetic energy of the steel platemember 110, which is a magnetic body.

During the course of disappearance of the closed loop C2, the change inmagnetic flux in the vicinity of the steel plate member 110 is detectedby the respective sensor coils 4 positioned in the vicinity of thesurface of the steel plate member 110. In an ideal situation, after thestatic magnetic field has been shut off, the change in magnetic fluxdetected by the sensor coils 4 decreases in a steady exponentialfashion. However, in practice, the situation deviates from this idealstate of change. This deviation is thought to be caused by transientcurrent which is induced in the steel plate member 110 by the change inthe state of magnetization in the steel plate member 110, during thecourse of disappearance of the magnetic energy (residual magnetic field)accumulated in the steel plate member 110. Therefore, the modeldescribed below can be hypothesized for the change in the magnetic fluxof the residual magnetic field after the static magnetic field has beenshut off.

FIG. 3 shows a model of the course of disappearance of the residualmagnetic field. In this course of disappearance, as illustrated in FIG.3, the density of the magnetic flux Φ passing through one of the sensorcoils 4 is denoted by B. Furthermore, the plurality of transientcurrents induced in the steel plate member 110 by the change in themagnetic flux density B are denoted by In1, In2, In3, . . . , and thecoefficient of induction relating to these induced transient currentsare denoted by M1, M2, M3, . . . The transient currents In1, In2, In3, .. . induced by the change in the magnetic flux density B are consideredto be mutually independent. Here, the transient currents In1, In2, In3,. . . can be substituted by a single transient current i2 induced by acoefficient of induction M=ΣMi (i=1, 2, 3, . . . ) in accordance withthe change in magnetic flux density B. Therefore, the course ofdisappearance of the magnetic flux Φ passing through any one sensor coil4, can be expressed by the magnetic flux density B, and the transientcurrent i2 induced by the coefficient of induction M, due to the changein magnetic flux density B.

FIG. 4 shows an equivalent circuit of FIG. 3. Here, R2 shows theelectrical resistance relating to the transient current i2, and L2 showsthe inductance relating to the transient current i2.

FIG. 5 is a diagram wherein the closed loop of magnetic flux Φ in thecircuit diagram in FIG. 4 (the closed loop C2 passing through a singlesensor coil 4 in FIG. 2) is substituted by an equivalent magneticcircuit. Here, i1 corresponds to the magnetic flux density (B in FIG.4). R1 corresponds to a given irreversibility of the magnetic flux Φ. L1corresponds to the inductance of the magnetic circuit, which is aphysical quantity directly proportional to the volume of the wholemagnetic flux space maintaining the magnetic flux Φ. Moreover, in thecircuit diagram shown in FIG. 5, the coefficient of induction Mcorresponds to the mutual inductance between the inductance L1 of themagnetic circuit and the inductance L2 of the transient current circuit.

FIG. 6 shows a schematic view of the closed loop C2 having magnetic fluxΦ (magnetic flux density B (=i1)) passing through a single sensor coil4, after the static magnetic field F1 has been shut off. As describedpreviously, after shut off of the static magnetic field, the magneticenergy W accumulated in the steel plate member 110 during theapplication of the magnetic field declines gradually, rather thandisappearing immediately. This magnetic energy W is maintained in theclosed loop space of magnetic flux Φ, and it gradually disappears inaccordance with the irreversibility R1 of the magnetic flux Φ. Ingeneral, the magnetic energy W can be expressed by the followingequation (1). $\begin{matrix}{W = {{\frac{1}{2\quad\mu}{\int{i_{1}^{2}{\mathbb{d}v}}}} = {\frac{1}{2}{Li}_{1}^{2}}}} & (1)\end{matrix}$

Here, L is a value that is directly proportional to the volume of thespace in which a magnetic flux of magnetic flux density i1 ismaintained, in other words, the volume of the space in which themagnetic energy is maintained. On the other hand, Equation (1) is thesame as the equation expressing the energy accumulated when a current ofi1 flows in a coil of inductance L. Therefore, it can be seen that theinductance L1 in the circuit diagram shown in FIG. 5 is directlyproportional to the volume of the total space in which the magnetic fluxis maintained.

The equivalent circuit shown in FIG. 5 can be represented by Equation(2). $\begin{matrix}\left. \begin{matrix}{{{L_{1}\frac{\mathbb{d}i_{1}}{\mathbb{d}t}} + {R_{1}i_{1}} - {M\frac{\mathbb{d}i_{2}}{\mathbb{d}t}}} = 0} \\{{{L_{2}\frac{\mathbb{d}i_{2}}{\mathbb{d}t}} + {R_{2}i_{2}} - {M\frac{\mathbb{d}i_{1}}{\mathbb{d}t}}} = 0}\end{matrix} \right\} & (2)\end{matrix}$

If the simultaneous differential equations indicated in Equation (2) aresolved for i1 and i2, then the following equations (3a) and (3b) areobtained as respective solutions. $\begin{matrix}{i_{1} = {{A_{1}\exp\left\{ {{- \left( {\alpha - \gamma} \right)}t} \right\}} - {A_{2}\exp\left\{ {{- \left( {\alpha + \gamma} \right)}t} \right\}}}} & \left( {3a} \right) \\{{i_{2} = {{A_{3}\exp\left\{ {{- \left( {\alpha - \gamma} \right)}t} \right\}} - {A_{4}\exp\left\{ {{- \left( {\alpha + \gamma} \right)}t} \right\}}}}{\alpha = {{A_{3}\exp\left\{ {{- \left( {\alpha - \gamma} \right)}t} \right\}} - {A_{4}\exp\left\{ {{- \left( {\alpha + \gamma} \right)}t} \right\}}}}{\gamma = \frac{\sqrt{\left( {{L_{1}R_{2}} - {L_{2}R_{1}}} \right)^{2} - {4R_{1}R_{2}M^{2}}}}{2\left( {{L_{1}L_{2}} - M^{2}} \right)}}{A_{1} = \frac{{- \left( {{L_{1}R_{2}} - {L_{2}R_{1}}} \right)} - \sqrt{\left( {{L_{1}R_{2}} - {L_{2}R_{1}}} \right)^{2} + {4R_{1}R_{2}M^{2}}}}{2R_{1}\sqrt{\left( {{L_{1}R_{2}} - {L_{2}R_{1}}} \right)^{2} + {4R_{1}R_{2}M^{2}}}}}{A_{2} = \frac{\left( {{L_{1}R_{2}} - {L_{2}R_{1}}} \right) - \sqrt{\left( {{L_{1}R_{2}} - {L_{2}R_{1}}} \right)^{2} + {4R_{1}R_{2}M^{2}}}}{2R_{1}\sqrt{\left( {{L_{1}R_{2}} - {L_{2}R_{1}}} \right)^{2} + {4R_{1}R_{2}M^{2}}}}}{A_{3} = \frac{- M}{\sqrt{\left( {{L_{1}R_{2}} - {L_{2}R_{1}}} \right)^{2} + {4R_{1}R_{2}M^{2}}}}}{A_{4} = A_{3}}} & \left( {3b} \right)\end{matrix}$

Here, the respective constants in equation (3a) and equation (3b) aredetermined for the initial conditions, wherein the magnetic flux densityi1 (=B) is taken to be B0 at the time that the static magnetic field F1is shut off (t=0). In this case, assuming that the coefficient ofinduction M is low and the transient current i2 induced by change in themagnetic flux density i1 is very small, in other words, L1·L2>>M·M, thenthe following results are obtained. $\begin{matrix}{{{\alpha - \gamma} \approx \frac{R_{1}}{L_{1}}} = \frac{1}{\tau_{1}}} & \left( {4a} \right) \\{{{\alpha + \gamma} \approx \frac{R_{2}}{L_{2}}} = \frac{1}{\tau_{2}}} & \left( {4b} \right) \\{A_{1} \approx I_{0} \approx {- \frac{1}{R_{1}}}} & \left( {4c} \right) \\{A_{2} \approx 0} & \left( {4d} \right) \\{A_{3} \approx 0} & \left( {4e} \right) \\{A_{4} \approx 0} & \left( {4f} \right)\end{matrix}$

If Equation (4a) and Equation (4b) are substituted into Equation (3a),and i1 is substituted for B, then equation (5) is obtained.B=A ₁exp(−t/τ ₁)−A ₂exp(−t/τ ₂)  (5)

Equation (5) indicates the transient change in the magnetic flux densityB of the magnetic flux Φ passing through the sensor coil 4. Here, takingEquation (4d) into account, it is possible to ignore the second item onthe right-hand side of Equation (5). Therefore, the change in themagnetic flux density B of the magnetic flux Φ forming the closed loopC2 shown in FIG. 6 can be approximated to the first item on theright-hand side of Equation (5) only. FIG. 7 a indicates the transientchange in the magnetic flux density B given by the first item on theright-hand side of Equation (5) only, after the time at which themagnetic field is shut off (t=0). The value of the magnetic flux densitybefore t=0 indicates the magnetic flux density B0 when the staticmagnetic field is being applied. On the other hand, the transientvoltage actually detected by a sensor coil 4 during the course ofdisappearance of the magnetic field is equal to the rate of change ofthe magnetic flux density B with respect to time, in other words, thedifferential magnetic flux density dB/dt, multiplied by thecross-sectional area of the magnetic flux passing through the sensorcoil 4. Therefore, by differentiating both sides of Equation (5) withrespect to time t, Equation (6), in other words, an equation for thedifferential magnetic flux density, can be derived. $\begin{matrix}\begin{matrix}{\frac{\mathbb{d}B}{\mathbb{d}t} = {{{- \frac{A_{1}}{\tau_{1}}}{\exp\left( {{- t}/\tau_{1}} \right)}} + {\frac{A_{2}}{\tau_{2}}{\exp\left( {{- t}/\tau_{2}} \right)}}}} \\{= {{- \frac{A_{1}}{\tau_{1}}}\left\{ {{\exp\left( {{- t}/\tau_{1}} \right)} - {\frac{A_{2}\tau_{1}}{A_{1}\tau_{2}}{\exp\left( {{- t}/\tau_{2}} \right)}}} \right\}}} \\{= {{- \frac{A_{1}}{\tau_{1}}}\left\{ {{\exp\left( {{- t}/\tau_{1}} \right)} - {\exp\left( {{- t}/\tau_{2}} \right)}} \right\}\left( {{{\because t} = 0},{\frac{\mathbb{d}B}{\mathbb{d}t} = {\left. 0\rightarrow\frac{A_{2}\tau_{1}}{A_{1}\tau_{2}} \right. = 1}}} \right)}} \\{= {{- \frac{B_{0}}{\tau_{1}}}\left\{ {{\exp\left( {{- t}/\tau_{1}} \right)} - {\exp\left( {{- t}/\tau_{2}} \right)}} \right\}\left( {\because{A_{1} \approx B_{0}}} \right)}} \\{= {{{- \frac{B_{0}}{\tau_{1}}}{\exp\left( {{- t}/\tau_{1}} \right)}} + {\frac{B_{0}}{\tau_{1}}{\exp\left( {{- t}/\tau_{2}} \right)}\left( {= {{f_{1}(t)} + {f_{2}(t)}}} \right)}}}\end{matrix} & (6)\end{matrix}$

FIG. 7 b shows the transient change in the differential magnetic fluxdensity obtained by Equation (6). This waveform is known to coincideapproximately with the waveform obtained when actual measurements aretaken used a sensor coil 4 as a magnetic sensor. Therefore, it can bemaintained that the model described with respect to FIG. 3 to FIG. 6 isan accurate reflection of the course of disappearance of the residualmagnetic field. In other words, Equation (5) represents the change inthe magnetic flux density B of the magnetic flux Φ passing through thesensor coil 4, and Equation (6) represents the differential change ofthe magnetic flux density dB/dt.

Here, it can be seen that term τ₁ in the first item on the right-handside of Equation (6) is equal to L1/R1, as can be deduced from Equation(4a), and therefore, it is equivalent to the time constant of themagnetic circuit of magnetic flux density i1 (=B) shown in FIG. 5.Consequently, the first item on the right-hand side of Equation (6)represents ideal steady attenuation characteristics wherein the magneticflux density B of the magnetic flux in the vicinity of the steel platemember 110 declines exponentially after the static magnetic field F1 hasbeen shut off, in other words, it represents the ideal attenuationcharacteristics of the magnetic energy. Here, it is supposed that thetime constant τ₁ contained in the first item on the right-hand side ofEquation (6) is called the “main time constant”. FIG. 7 c shows theattenuation characteristics of the magnetic energy expressed by thefirst item on the right-hand side of Equation (6) (the closed loop C2 ofthe magnetic flux Φ).

In order to determine the main time constant τ₁, firstly, Equation (7)is obtained by taking the logarithm of either side of f1(t) expressed bythe first item on the right-hand side of Equation (6). If Equation (7)is plotted on a graph, a straight line of gradient 1/τ₁ is obtained.Therefore τ₁ can be determined from the gradient of the graph.$\begin{matrix}{{{In}{\quad\quad}{f_{1}(t)}} = {{{- {In}}\frac{B_{0}}{\tau_{1}}} + \frac{t}{\tau_{1}}}} & (7)\end{matrix}$

In the manner described above, it is possible to determine the main timeconstant of the transient change in the differential magnetic fluxdensity of the residual magnetic field which disappears after the staticmagnetic field has been shut off.

The second term τ₂ in the right-hand side of Equation (6) is equal toL₂/R₂ as will be understood from Equation (4b), and thus corresponds tothe time constant of the equivalent circuit of the eddy current i₂ inFIG. 5. Therefore, the second term in the right-hand side of Equation(6) expresses the damping characteristic of eddy current loss. FIG. 7 dshows the damping characteristic of the eddy current loss represented bythe second term of the right-hand side in Equation (6). Hereafter, thetime constant τ₂ of the second term of the right-hand side of Equation(6) will be called the secondary time constant. The secondary timeconstant τ₂ can be obtained in the same way as described for τ₁.

The main time constant τ₁ obtained as described is proportional to L₁.On the other hand, L₁ is proportional to the magnetic permeability μ ofthe magnetic path through which the magnetic flux Φ passes. These factsshow that the main time constant τ₁ is proportional to the magneticpermeability μ. It should also be noted that the nugget section 102, thepressure bonded section 103 and the HAZ 104 of the spot weld sectionhave different metallographic structures and therefore differentmagnetic permeabilities μ. Specifically, it is known that the magneticpermeability μ decreases as the hardness increases.

The non-destructive inspection method disclosed in the Japanese PatentApplication Laid-Open No. 10-26209 uses the measuring principlesdescribed above in obtaining the main time constant τ of the magneticenergy damping characteristic at each location of the sensor coils 4,and uses the distribution of the main time constant τ₁ as the basis ofestimation of the nugget diameter L2 of the nugget section 102 which isthe hardest in the spot weld section and therefore has the lowestmagnetic permeability μ.

Specifically, first, as shown in FIG. 8 a, a row of e.g. sixteen sensorcoils 4 are arranged to face the spot weld section and its surroundingsfor measurement of the damping characteristic of the magnetic flux Φ inthe residual magnetic field by each of the sensor coils 4. From thismeasurement data, the main time constant τ₁ is obtained for each of thesensor coils 4. As shown in FIG. 8 b, the distribution of the τ₁ for therespective sensor coils 4 can be expressed by a step function with themeasurement position x as the variable. Next, as shown in FIG. 8 c, anapproximate curve f(x) (solid line) of the step function is calculatedby the least squares method. Supposing that only the nugget section 102has a relatively low magnetic permeability, and all the other regionshave the same magnetic permeability, the recess in the approximate curvef(x) for the τ₁ distribution reflects the existence and the geometry ofthe nugget section 102, for τ₁ is proportional to the magneticpermeability. Thus, according to the technique disclosed in the JPLaid-Open No. 10-26209, the shape of the recess in the approximate curvef(x) for the τ₁ distribution is used for estimating the nugget diameterL2 as shown in FIG. 8 c.

As discussed earlier, the damping characteristic main time constant τ₁of the magnetic flux Φ through the sensor coil 4 is proportional to theinductance L₁, and L₁ is proportional to the magnetic permeability μ. Itis also known that L₁ is proportional to the volume V of the spacethrough which the magnetic flux Φ passes. Thus, the relationshipexpressed in Equation (8) holds for the main time constant τ₁:τ₁ ∝μV  (8)

As noted above, air has an extremely smaller magnetic permeability thanany of the magnetic matters in the spot weld section. For this reason,the τ₁ calculated for the measurement positions facing the indentations101 in FIG. 18 is smaller than the counterpart values for othermeasurement positions, whether or not the nugget section 102 exists. Inother words, the recess in the graph in FIG. 8 c includes a componentresulted from the indentations 101. If the spot weld section were flatwith no indentations 101 formed, the τ₁ distribution approximate curvef(x) should have a shallower recess as shown by the broken line in FIG.8 c. Thus, if the non-destructive inspection method of a spot weldsection utilizes only one approximate curve f(x) of the distribution ofτ₁, it may be difficult to obtain an accurate estimation for the nuggetdiameter L2 due to the noise caused by the presence of the indentations101.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view outlining a constitution and operation of anapparatus for carrying out a non-destructive inspection through turningon and shutting off a magnetostatic field.

FIG. 2 is a schematic view outlining another constitution and operationof an apparatus for carrying out a non-destructive inspection throughturning on and shutting off a magnetostatic field.

FIG. 3 shows a model of fading process of magnetic flux closed loopsafter a magnetostatic field is shut off.

FIG. 4 shows an equivalent circuit of an eddy current shown in FIG. 3.

FIG. 5 shows an equivalent to FIG. 4, with a close loop of magnetic fluxdensity replaced by a magnetically equivalent circuit.

FIG. 6 shows a magnetic flux closed loop passing through a coil rightafter a magnetostatic field is shut off.

FIG. 7 a through FIG. 7 d show transient changes of different physicalquantities in non-destructive inspection according to the presentinvention.

FIG. 8 a through FIG. 8 c illustrate a concept of an example method ofmeasuring a nugget diameter.

FIG. 9 shows a non-destructive inspection apparatus capable of carryingout a non-destructive inspection method according to the presentinvention.

FIG. 10 is a sectional view taken in lines X-X in FIG. 9.

FIG. 11 shows a configuration of a system for operating thenon-destructive inspection apparatus in FIG. 9.

FIG. 12 show how a main time constant τ₁ changes in differentmetallographic structures in a spot weld section, along with changes inmagnetic flux density B of an applied magnetostatic field.

FIG. 13 is a flowchart of a first embodiment.

FIG. 14 a through FIG. 14 c illustrate a concept of non-destructiveinspection method according to the first embodiment.

FIG. 15 shows an example of correlation data obtained by the presentinvention.

FIG. 16 is a flowchart of a second embodiment.

FIG. 17 a through FIG. 17 c illustrate a concept of non-destructiveinspection method according to the second embodiment.

FIG. 18 is an illustration of spot welding.

FIG. 19 is a sectional view of a spot weld section made by spot weldingtwo sheet metals.

BEST MODE FOR CARRYING OUT THE INVENTION

FIG. 9 shows a non-destructive inspection apparatus X for carrying out anon-destructive inspection method according to the present invention.FIG. 10 is a sectional view taken along lines X-X in FIG. 9, showing thestate of the magnetostatic field formed. The non-destructive inspectionapparatus X performs a non-destructive inspection based on theabove-described measuring principles which involve the applying andshutting-off of the magnetostatic field. The non-destructive inspectionapparatus X includes an exciting pole 11, an excitation coil 12 woundabout the exciting pole, a recovering pole 13, a connecting portion 14to connect the exciting pole 11 and the recovering pole 13, and a coilarray 15 disposed in the vicinity of the exciting pole 11.

The exciting pole 11 is an iron core for increasing the magnetic fluxdensity of the magnetic field which is induced by a current flowingthrough the excitation coil 12, and is integral with the recovering pole13 via the connecting portion 14. The exciting pole 11 has a magneticflux exciting surface 11 a at its end. The recovering pole 13 has arecovering surface 13 a at its end. Magnetic fluxes coming from themagnetic flux exciting surface 11 a of the exciting pole 11 arerecovered by the recovering surface 13 a.

The coil array 15, provided by a row of loop coils 15 a according to thepresent embodiment, detects magnetic changes near an inspection regionduring the application and after the shutoff of the magnetostatic field,and outputs the measured changes in the form of voltage. Each loop coil15 a is made of an electrically conductive material such as copper, andis patterned on a flexible substrate 16. As shown in FIG. 9, the coilarray 15 is right beneath the magnetic flux exciting surface 11 a,spaced from the magnetic flux exciting surface 11 a by a predetermineddistance, so that the row of loop coils 15 a lies longitudinally of themagnetic flux exciting surface 11 a. The coil array 15 is placed in sucha relationship to the magnetic flux exciting surface 11 a of theexciting pole 11 that the loop coils 15 a are displaced toward therecovering pole 13. Such a construction makes it possible to efficientlymeasure the magnetic flux of the magnetostatic field headed toward therecovering surface 13 a and measure the residual magnetic fieldgenerated when the magnetostatic field is shut off.

FIG. 11 shows the composition of a system for operating thenon-destructive inspection device X1. This system comprises a sensoroperating section 20, a control section 30 and a data processing section30.

The sensor operating section 20 comprises a sensor output switchingsection 21 and a buffer amp 22. The sensor output switching section 21is a circuit for selecting only one output of the respective outputs ofthe plurality of loop coils 15 a constituting the coil array 15, andoutputting same to the buffer amp 22. The sensor output switchingsection 21 selects the outputs of the respective loop coils 15 a,sequentially, and outputs same to the buffer amp 22, in accordance witha 4-bit sensor output switching signal S1. The buffer amp 22 is a buffercircuit for outputting the output signal from the sensor outputswitching section 21 to the control section 30, in the form of adetection signal S2.

The control section 30 is formed by a control circuit connected via ageneric bus 38 to the data processing section 40, and it comprises asensor control section 30 a and a signal processing section 30 b. Thesensor control section 30 a comprises an excitation control section 31and a sensor output control section 32. The signal processing section 30b comprises a waveform adjusting section 33, an A/D converter 34, a dualport memory 35, an A/D controller 34 a, and a memory controller 35 a.The control section 30 is formed on a control substrate connected to ageneric slot of a computer.

The excitation control section 31 of the sensor control section 30 aoutputs a drive signal S3 to the excitation coil 20, in order togenerate or shut off a static magnetic field of a prescribed intensity.In other words, the excitation control section 31 applies and shuts offa prescribed voltage, to the excitation coil 12. The sensor outputcontrol section 32 outputs a 4-bit sensor output switching signal S1 forsequentially selecting the output from the plurality of loop coils 15 acontained in the coil array 15, to the sensor output switching section21.

The waveform adjusting section 33 of the signal processing section 30 badjusts the detection signal S2 from the buffer amp 22, in accordancewith the input specification of the A/D converter 34. The A/D converter34 converts the input detection signal from analog to digital. The dualport memory 35 stores the digital data after A/D conversion. The A/Dcontroller 34 a controls the timing of the A/D converter 34. The memorycontroller 35 a controls the operations of writing to and reading fromthe dual port memory 35.

If the inspection target is a steel plate, then the effects of the eddycurrent loss appear notably in the transient change characteristics ofthe detection signal S2 after approximately 10 μs or less from the shutoff of the static magnetic field (the average value being approximately3 to 6 μs). Taking account of this fact, and the accuracy of dataprocessing, it is desirable that the A/D converter 34 has a conversionspeed of 4 Msps or above, and a conversion accuracy of 12 bits, or more.

The data processing section 40 is realized by means of a generalcomputer having non-illustrated CPU and main memory. In the dataprocessing section 40, the nugget diameter of the spot weld section isfound by processing detection data which is output by the sensoroperating section 20 and processed by the signal processing section 30b. The data processing section 40 comprises a monitor for displayingvarious measurement waveforms and measurement data tables. The varioustypes of data processing described hereinafter are achieved by executinga computer program stored in a main memory (storage medium) by a CPU inthe data processing section 40.

With the non-destructive inspection apparatus X and the operating systemdescribed above, it is possible to carry out a non-destructiveinspection method according to the present invention.

FIG. 12 shows how main time constants τ₁ corresponding to differentmetallographic structures in a spot weld section will change as themagnetic flux density B of an applied magnetostatic field changes. Thegraph shown in FIG. 12, representing the changes of the main timeconstants τ₁, is obtained by using the above-described measurementprinciples applied to individually-prepared metallic samples which arearranged to simulate, respectively, the nugget section 102, the pressurebonded section 103, the HAZ 104 and the original metal 105, which aredisposed at or near the spot weld section shown in FIG. 19. The curves41, 42, 43 and 44 correspond to the nugget section 102, the pressurebonded section 103, the HAZ 104 and the original metal 105,respectively.

As shown in FIG. 12, the main time constants τ₁ tend to be greater forthe nugget section 102, the pressure bonded section 103, the originalmetal 105 and the HAZ 104 in this order for the same magnetic fluxdensity B. This is because, as noted above, the hardness of the nuggetsection 102, the pressure bonded section 103, the original metal 105 andthe HAZ 104 becomes smaller in this order, while their magneticpermeability μ, which is proportional to the main time constant τ₁,become greater along with the decrease in hardness. The main timeconstants τ₁ for the respective sections are generally constant or showgentle linear changes; but in a relatively narrow range of the magneticflux density, they increase fairly sharply, thereby providing a peak.When measurements are made under the same conditions, the peak of eachcurve appears at a given magnetic flux density B specific to thehardness of the section. Another tendency is that, in the illustratedrange of the magnetic flux density, the specific value of the magneticflux density B shifts toward the higher value side as the hardnessbecomes smaller.

The non-destructive inspection method according to the first embodimentof the present invention will be described with reference to FIG. 12 andFIG. 14 as well as the flowchart of FIG. 13. Specifically, in Step 1,the non-destructive inspection apparatus X is disposed so that a row ofsixteen loop coils 15 a serving as magnetic sensors are arranged to facethe spot weld section and its surroundings. The locations of the loopcoils 15 a are represented by the measurement position x. Next, in StepS2, the non-destructive inspection apparatus X applies a magnetostaticfield to the spot weld section, at a magnetic flux density B1.Preferably, referring to FIG. 12, the magnetic flux density B1 isgreater than the magnetic flux density giving rise to the peak in the τ₁curve of the nugget section 102, but smaller than the magnetic fluxdensity giving rise to the peak in the τ₁ curve for the pressure bondedsection 103. Next, in Step S3, the magnetostatic field is shut off.Next, in Step S4, the loop coils 15 a measure the disappearance of theresidual magnetic field for the spot weld section and its surroundings.Next, in Step S5, based on the measurements, the time constant τ₁ ateach measurement position is analyzed. Next, in Step S6, the values ofthe time constant τ₁ for each sensor location, or measurement positionx, are plotted, and than an approximate curve 61 as shown in FIG. 14 bis calculated from the plot data by means of the least squares method.The approximate curve 61 may be displayed on the monitor of the dataprocessing section 40, as necessary.

Next, the process goes back to Step S2 of the flowchart in FIG. 13,whereby the non-destructive inspection apparatus X, held at the initialposition, applies a magnetostatic field again to the spot weld sectionand its surroundings, at a magnetic flux density of B2. As shown in FIG.12, preferably the magnetic flux density B2 gives rise to the peak inthe curve for the pressure bonded section 103, but not in any othercurves for the other sections. Next, in Step S3, the magnetostatic fieldis shut off. In Step S4, the respective loop coils 15 a measure thedisappearance of the residual magnetic field for the spot weld sectionand its surroundings. Based on the measurements, in Step S5, the maintime constants τ₁ at each measurement position are analyzed. In Step S6,the main time constants τ₁ for the respective sensor locations ormeasurement position x are plotted. From the plot data, an approximatecurve 62 as shown in FIG. 14 b is calculated by the least squaresmethod. The approximate curve 62 may be displayed on the monitor of thedata processing section 40, as necessary.

Each of the magnetic paths corresponding to the positions of therespective loop coils 15 a, i.e., the measurement positions, extendsthrough various portions constituting the spot weld section. Asunderstood from FIG. 12, the time constant τ₁ measured at eachmeasurement position with the magnetic flux density B1 will be the sameas the time constant τ₁ measured at each measurement position with themagnetic flux density B2 if the magnetic paths corresponding to therespective measurement positions do not pass through the pressure bondedsection 103. On the other hand, if most part of the magnetic pathextends through the pressure bonded section 103, the time constant isgreater when measured by the magnetic flux density B2 than when measuredby the magnetic flux density B1. This difference is reflected in theoccurrence of the peaks in the ratio curve 63 shown in FIG. 14 c. Ineach of the approximate curves 61, 62 in FIG. 14 b, a recess includes acomponent originated from the indentations 101, but the ratio curve 63in FIG. 14 c, which shows changes in the ratio between the two curves,does not includes the component from the indentations 101. Therefore,the present embodiment makes it possible to obtain information about theinternal structure appropriately regardless of the outside surfaceundulation in the spot weld section.

FIG. 14 c shows a case in which the nugget diameter L2 is estimated tobe the distance between the two peaks of the ratio curve 63. As tointerpretation of a specific point in the peak to be the border betweenthe nugget section 102 and the pressure bonded section 103, it ispreferable that the interpretation should be made through apredetermined set of steps, on the basis of correlation data between theestimated nugget diameter L2 obtained from the peak data of the ratiocurve 63 according to the present embodiment and actual measurement dataof the nugget diameter L2.

FIG. 16 shows a correlation data between estimated values of the nuggetdiameter L2 given by the peak-to-peak distance in the ratio curve 63 andvalues obtained from actual measurements of the nugget diameter L2. Thecorrelation data yields a correlation coefficient of about 0.92,indicating that the estimated values are highly accurate.

According to the present embodiment, it is also possible to obtain otherinformation such as diameters of other zones, by selecting twoappropriate values based on the τ₁ change curve data as shown in FIG. 12for the magnetic flux density B of the applied magnetostatic field.

Next, a second embodiment of the present invention will be describedwith reference to FIG. 12, FIGS. 17 a-17 c and the flowchart in FIG. 16.First, in Step 1′, as shown in FIG. 17 a, the non-destructive inspectionapparatus X is placed so that the row of sixteen loop coils 15 a servingas magnetic sensors are arranged to face a spot weld section and itssurroundings. The locations of the loop coils 15 a are represented bymeasurement positions x. Next, in Step S2′, the non-destructiveinspection apparatus X applies a magnetostatic field to the spot weldsection, at a magnetic flux density B3. As shown in FIG. 12, themagnetic flux density B3 ideally has a value smaller than a magneticflux density which brings the time constant to the peak in the curve forthe nugget section 102, but not in any other curves for the otherregions. Next, in Step S3′, the magnetostatic field is shut off. Next,in Step S4′, measurements are made by each loop coil 15 a for a fadingprocess of a residual magnetic field at the spot weld section and itssurroundings. Next, in Step S5′, analyses are made for the time constantat each position of measurement, based on this particular cycle ofmeasurements. In this step, analytical calculation is also made for avalue of the magnetic flux density at t=0 which gave this particular setof main time constants. Specifically, the value is obtained by the dataprocessing section 40 through an integrating operation from t=0 to t=∞of the first term in the right-hand side of Equation (6) thatcorresponds to FIG. 7 c. The main time constants and the correspondingmagnetic flux densities B are stored in the main memory in the dataprocessing section 40. Next, in Step S6′, plotting is made for values ofthe time constant for each sensor location or measurement position x,and from the plot data is obtained an approximate curve 80 as shown inFIG. 17 b by means of the least squares method. The approximate curve 80is displayed on the monitor of the data processing section 40, asnecessary. Step S6′ may be skipped according to the present embodiment.It should be noted that the approximate curve 80 in FIG. 17 b isschematic for simplicity of the drawing.

Next, the process goes back from Step S5′ or S6′ to Step S2′ of theflowchart in FIG. 16, and a cycle of steps from Step S2′ through StepS5′ or S6 is repeated for n times, with the magnetic flux density B ofthe magnetostatic field applied by the non-destructive inspectionapparatus X incremented by ΔB for each cycle. A value of the magneticflux density in the n-th cycle is B4. As shown in FIG. 12, the magneticflux density B4 is ideally greater than a magnetic flux density whichbrings the time constant to the peak in the curve for the HAZ 104, butnot in the curves for any other zones. As described, according to thepresent embodiment, the magnetic flux density B of the appliedmagnetostatic field is varied, or changed for scanning, whereby changesin the main time constant at each measurement position is scanned.

After the scanning is finished, in Step S7′, reference is made to themain memory in the data processing section 40, to identify for eachmeasurement position a magnetic flux density value of a criticalmagnetostatic field where the time constant showed dramatic increase inresponse to the change in the magnetic flux density B. As will beunderstood from FIG. 12, each of the regions constituting the spot weldsection and its surroundings has a specific magnetic flux density valuefor the critical magnetostatic field where the time constant showsdramatic increase, in a range between the magnetic flux density B3 atthe beginning of scanning and the magnetic flux density B4 at the end ofthe scanning. At each position where one of the loop coils 15 a islocated, i.e. at each position of the magnetic path represented by oneof the measurement positions, there is a complex of different zones thatconstitute the spot weld section. The variation in the main timeconstant at each measurement position is a significant representation ofan influence by the structure (zone) which takes a primary or thelargest volume at this particular position of the magnetic path. FIG. 17b gives a schematic view of different waveforms (approximate curves) ofthe main time constant at each magnetic flux density in a case thescanning was made at a total of 10 different values of the magnetic fluxdensity B. Arrows in the figure indicate the positions x at which adramatic increase of the τ₁ was identified.

Next, in Step S8′, a magnetic flux density curve 90 with the measurementposition x as a variable is obtained as shown in FIG. 17, based on themagnetic flux density data of the critical magnetostatic fieldidentified in Step S7′ where the time constant showed dramatic increase.The magnetic flux density curve 90 in FIG. 17 is a two-dimensionalinterpretation of the internal structure information of the spot weldsection. Specifically, x1-x6 represents the HAZ diameter L4, x2-x5represents the joint diameter L3, and x3-x4 represents the nuggetdiameter L2. The magnetic flux density curve 90 is displayed asnecessary on the monitor of the data processing section 40.

Next, in Step S9′, the lengths of x3-x4, x2-x5 and x1-x6 are calculatedbased on the data regarding the magnetic flux density curve 90, wherebya nugget diameter L2, a joint diameter L3 and a HAZ diameter L3 aregiven estimated values. Next, in Step S10′, the estimated nuggetdiameter L2 is compared to a predetermined threshold value, to determineon the quality of welding of the spot weld section.

Each of the approximate curves 80-89 in FIG. 17 b includes a componentoriginated from the indentations 101. However, the magnetic flux densitycurve 90 in FIG. 17 c is plotted in the coordinate system with thevertical axis representing the magnetic flux density that gives dramaticchange in these curves, and does not include a component originated fromthe indentations 101. Therefore, the present embodiment makes itpossible to obtain information about the internal structureappropriately regardless of the outside surface undulation in the spotweld section. It should be appreciated that FIG. 17 b also showsmagnetic flux density curves in broken lines, each based on the magneticflux density distribution data at t=0 for each of the measurementpositions, calculated in Step S5′ in the cycles with the magnetic fluxdensity being B3 through B4. Each of these magnetic flux density curverepresents the surface waveform of the spot weld section, and thesecurves do not differ from each other throughout the cycles, indicatingthat the indentations 101 does not affect the magnetic flux densitycurve 90 in FIG. 17.

FIG. 17 c shows a case in which the nugget diameter L2 is estimated tobe the distance between the two inflection points of the magnetic fluxdensity curve 90 at the lowest magnetic flux density. As tointerpretation of a specific point in the inflection to be the borderbetween the nugget section 102 and the pressure bonded section 103, itis preferable that the interpretation be made through a predeterminedset of steps, on the basis of correlation data between the estimatednugget diameter L2 obtained from the inflection point data of themagnetic flux density curve 90 according to the present embodiment andactual measurement data of the nugget diameter L2.

In the first and the second embodiments, the results of measurements arenot influenced by the indentations 101. As a result, it has becomepossible to make accurate estimation on the nugget diameter L2, andtherefore to obtain highly reliable inspection results on the weldingquality of spot weld sections. Further, it is also possible to obtaindetailed information on the internal structure of the spot weld section,such as the joint diameter L3 and the HAZ diameter L4.

The present invention has been described above by taking an example ofnon-destructive inspection method for spot weld sections. The presentinvention is not limited to this, and is applicable also tonon-destructive inspection apparatuses and methods for measuring andinspecting internal flaws, hardness, acting stresses and so on in steelmembers for example. The non-destructive inspection method according tothe present invention has been described with reference to graphs andother visual representations, but it should be noted that various dataprocessing and analyses can be made by means of arithmetic processingusing a variety of mathematical functions equivalent to each of thegraphs exhibited.

1. A non-destructive inspection method including: a step of magnetizingan inspection target by applying a first magnetostatic field to thetarget; a step of shutting off the first magnetostatic field andmeasuring transient change in a differential magnetic flux density of afirst residual magnetic field passing through the magnetized target, themeasuring being performed at a plurality of measurement positions; astep of obtaining a first time constant provided by a main time constantof the transient change for each of the measurement positions; a step ofmagnetizing the target by applying a second magnetostatic field to thetarget; a step of shutting off the second magnetostatic field andmeasuring transient change in a differential magnetic flux density of asecond residual magnetic field passing through the magnetized target,the measuring being performed at each of the measurement positions; astep of obtaining a second time constant provided by a main timeconstant of the transient change for each of the measurement positions;and an information obtaining step of obtaining information about aninternal structure of the target based on a difference betweendistribution of the first time constant and distribution of the secondtime constant at the measurement positions.
 2. The method according toclaim 1, wherein the measurement positions are in a row facing thetarget.
 3. The method according to claim 1, wherein the informationabout the internal structure in the information obtaining step isobtained based on a ratio function which is derived from a distributionfunction of the first time constant with the measurement position as avariable and a distribution function of the second time constant withthe measurement position as a variable.
 4. The method according to claim1, wherein the information about the internal structure in theinformation obtaining step is obtained based on a difference functionwhich is derived from a distribution function of the first time constantwith the measurement position as a variable and a distribution functionof the second time constant with the measurement position as a variable.5. The method according to claim 1, wherein the target is a spot weldsection in a jointed plate member made by spot welding two sheet metals.6. The method according to claim 5, wherein the information obtained inthe information obtaining step includes information about a shape ofnugget section included in the spot weld section.
 7. A non-destructiveinspection method comprising: a scanning step including a cycle ofmagnetizing a target by applying a magnetostatic field to the target,shutting off the magnetostatic field to measure transient change in adifferential magnetic flux density of a residual magnetic field whichpasses through the magnetized target at a plurality of measurementpositions, and obtaining a main time constant of the transient changefor each of the measurement positions, the cycle being repeatedlyperformed for each of a plurality of magnetostatic fields of differentmagnetic flux densities; an analyzing step of analyzing a change thatthe main time constant undergoes at each measurement position as theplurality of magnetostatic fields are changed in the scanning step; andan information obtaining step of obtaining information about an internalstructure of the target based on an analysis result obtained by theanalyzing step.
 8. The method according to claim 7, wherein in theanalyzing step, at each of the measurement positions, a magnetic fluxdensity of a critical magnetostatic field is determined, for which fieldthe change of the main time constant during the scanning step withrespect to changes in the magnetostatic field achieves a maximum value,and wherein in the information obtaining step, information about theinternal structure of the target is obtained based on a distributionfunction of the critical magnetostatic field with the measurementpositions as a variable.
 9. The method according to claim 7, wherein themeasurement positions are in a row facing the target.
 10. The methodaccording to claim 7, wherein the target is a spot weld section in ajointed plate member made by spot welding two sheet metals.
 11. Themethod according to claim 10, wherein the information about the internalstructure obtained in the information obtaining step comprisesinformation about a shape of a nugget section included in the spot weldsection.